In holographic data storage the user data is usually modulated in two-dimensional data pages consisting of pixels with the binary state of either 0 or 1. In order to record data a spatial light modulator (SLM) transforms a user data pattern into an intensity modulation of an object beam. This object beam is superimposed with a reference beam, which results in an interference pattern. This pattern is recorded into a holographic storage medium, e.g. a holographic disk or card. During the reading process the reference beam (usually called probe beam) is diffracted by the interference pattern. This results in a reconstructed object beam holding the stored information. By imaging the reconstructed object beam onto a photo sensitive detector the data page is recovered.
In an optimum case the reconstructed data page has a uniform intensity profile where all regions exhibit the same pixel contrast. However, due to several reasons this is usually not the case. One reason is an inhomogeneous material sensitivity, e.g. due to the chemical properties of the employed holographic polymers. Regions already irradiated by other holograms, e.g. in the case of shift multiplexing, may have a different sensitivity than not yet irradiated regions. The difference in sensitivity depends on the sensitivity curve of the material. Because of this effect it may happen that parts of the holographic interference pattern exhibit a higher contrast than others. This results in a non uniform contrast of the recovered data page. Contrast in a region is to be understood as the difference between the mean intensity of data pixels in the state ‘0’ and the mean intensity of data pixels in the state ‘1’ in this region. A high contrast means that the difference is large.
For a reliable pixel detection it should be ensured that the low-contrast parts of the recovered data page are bright enough for pixel detection, whereas the high-contrast parts should not exceed the dynamic range of the detector. Furthermore, as the bright parts exhibit a higher brightness than necessary for detection, a fraction of the available beam energy is wasted.